Thin film filters (“TFFs”) are manufactured by depositing many, possibly hundreds of layers or films of varying thickness onto a glass substrate. The thickness, refractive index, number and sequence of the layers determine the effect of the filter on incident light, applied in a direction normal to the filter layers. High-pass, low-pass, band-pass and other filter types can be realized.
TFFs are conventionally fabricated using wafer-processing techniques. Each filter may have a planar dimension of 1–2 mm on a side, resulting in thousands of filters on, for example, a 6 inch wafer. Among the difficult issues confronted during fabrication are dicing the wafers post-deposition into individual TFF dies, and then optically testing each die. In its ultimate position in a functioning system, the alignment of a TFF is carefully controlled to be both axially aligned with, and perpendicular to, the incident light source. A similar positioning accuracy must also be attained during post-fabrication optical testing of the TFFs, to ensure their accurate optical characterization.
Other optical devices, such as diffractive optics, micro-lens arrays (small, monolithic, lithographically fabricated lenses), or diffraction grating arrays have similar alignment and test problems.
Currently, the fabrication and test process involves mounting the finished wafer onto a smooth, planar plate; dicing through the wafer and partially into the plate according to a dicing pattern resulting in many individual die; removing the individual TFF die from the plate; affixing each to a carrier; and then cleaning each individual die. Each die thus requires re-collecting into a carrier for subsequent presentation into the test equipment. The test equipment must handle small individual dice and their movement into and out of their test positions, including precision alignment centered on, and perpendicular to, the incident light source.
One major problem with this approach is the need to sort and position each individual die for testing. This process consumes resources including test time and extra equipment. The exposure of the individual dice to this extra handling may also affect the integrity of the dice; there is more likelihood of chipping and scratching (which decreases overall yield).
The process of individual alignment may also lead to testing inaccuracies and/or inconsistencies. There is no sure way to guarantee a consistent test position among individual die since they are individually positioned. Moreover, the small size of each die leads to positioning inaccuracies, especially when aligning the die surface to be perpendicular to the test source. Handling individual die is unproductive as it forces the use of sophisticated machine vision systems, and additional motions, to locate the position of each die, thereby adding considerable cost and complexity, and reducing test throughput.
What is required, therefore, is a technique for fabrication, handling and test of optical devices such as TFFs, which improves process throughput, yield, and test accuracy.